Manufacturing a container by blowing ordinarily consists of inserting, into a mold with the imprint of the container, a blank (a preform or an intermediate container obtained by pre-blowing a preform) previously heated to a temperature above the glass transition temperature of the material, and of injecting into the blank a fluid (particularly a gas such as air) under pressure. The blowing can be completed by a preliminary stretching of the blank by means of a sliding rod.
The dual molecular orientation that the material undergoes during blowing (axial and radial, respectively parallel and perpendicular to the general axis of the container) gives a certain structural rigidity to the container.
However, the reduction—dictated by the market—in the quantity of material used for the manufacture of containers leads manufacturers to resort to artifices of manufacturing or shape to rigidify their containers since the bi-orientation proves to be insufficient. The result is that two containers of equal weight do not necessarily have the same mechanical performance (strength, rigidity).
A well-known method of increasing the rigidity of the container is heat setting, which consists of heating the wall of the mold in order to increase the rate of crystallinity by means of heat. This method, illustrated by French patent FR 2,649,035 (Sidel) and its American equivalent U.S. Pat. No. 5,145,632, is used particularly for heat resistant (HR) applications in which the container is hot-filled.
However, because of its cost and the reduction in production rate it requires, this type of method generally cannot be used for ordinary applications such as flat water. For these applications, the structural rigidity of the bottom essentially depends on its shape. It is known to rigidify the bottom by means of radial grooves, see for example French patent FR 2,753,435 (Sidel). Such a bottom preserves its mechanical strength without reversing, as long as the volume and pressure conditions in the container are normal. However, when these conditions are extreme, the bottom tends to collapse or even reverse. Thus, when the container is stored in high heat, typically when it is stored on a pallet outdoors in full sun, the temperature of the contents can reach or exceed 50° C., and the increase in pressure caused by the expansion of the contents exceeds the threshold beyond which the bottom reverses. The container then becomes unstable, with the increased risk of collapse of the whole pallet. Similarly, when the container is stored in a cooler at temperatures at which the contents freeze, the expansion induced by the solidification causes the bottom to reverse, the container also becoming unstable.
In theory, it would be possible to form deep hollow reserves on the bottom (particularly a deep arch) that could increase the mechanical strength of the bottom. However, this artifice, although effective, requires both an increase in material, incompatible with the aforementioned lightening requirements, and a high blowing pressure, incompatible with energy-saving requirements, which on the contrary assume a decrease in the blowing pressure needed for forming the container. For example, the current specifications for forming bottles for flat water with a capacity of 0.5 liter ordinarily require a mass on the order of 10 g, for a blowing pressure on the order of 20 bars.